"Done right" is a tall tale. Anything can be shaped into usefullness.Done right, these things complement one another.
Having an erotic story that has protracted segments that have nothing to do with sex is called "losing focus". Do you want them to masturbate or do you want your work to have literary merit? Both are cool, but pick one. Wanting to pick both is immature.
@oggbashan
If the whole point is arousal, by whatever device, - it's erotica. If it lists genitalia and their use, it's a subtype of erotica: porn. Gradations of subtlety are irrelevant for the overall definition.